The invention relates to an insert for installation in a relatively thin or weak workpiece to provide an anchor receptacle for a self-tapping screw or the like.
Self-tapping screws are widely used in industry as providing a means of securing together two components which is inexpensive in respect of both manufacturing cost and installation time. However, their use has a number of disadvantages, stemming from the material or receptacle into which they are inserted.
When a self-tapping screw is driven directly into a workpiece of thin or weak material, the failure strength of the thread formed in the workpiece is low. Further, there is little margin between this and the torque that is required to start the thread engagement. Thus it can be difficult to control the fastener installation torque so as to avoid thread stripping. Also, the tension strength of the resultant joint is limited by the tensile strip-out force of the thread in the thin or weak material.
A grommet or insert of plastics material (e.g. nylon) is sometimes used as a receptacle for a self-tapping screw. However, since plastics material is relatively weak, it requires a long length of screw engagement to provide enough strength to create sufficient margin between the screw-engagement torque and the thread strip-out torque so as to enable adequate control of the operation in a practical industrial environment, such as a car assembly line. The long length of the screw engagement produces a large protrusion on the rear or blind side of the workpiece, which may be awkward or dangerous in the use of the assembled product. The longer screw also takes more time to instal.
Furthermore since the insert or anchor is of totally different material to the workpiece (e.g. plastics as against steel), ultimate scrapping of the assembled product (e.g. a car body) makes it difficult to recover both materials. Plastics components are usually burnt off and the material not recovered.
One possible alternative would be to use a blind threaded insert, such as is widely known and available in many countries under the name AVDEL THIN SHEET NUTSERT (Registered Trade Marks) in conjunction with a machine screw or bolt. This can have drawbacks under production-line conditions where speed of operation is under pressure, such as difficulty of engaging threads, leading to cross-threading. Further, if the component with the installed threaded insert is painted before the screw is applied, the paint can enter the insert and jam the threads thus preventing proper engagement of the screw.
One of the aims of the present invention is to provides an insert which overcomes these practical disadvantages of the prior art. The invention is defined in the accompanying claims.